The use of ultrasonic imaging for medical diagnostic purposes is well known. In particular, ultrasound has been used for many years to aid in the diagnosis of certain cardiac diseases. In addition, cardiac Doppler ultrasound technology has become recognized as an important tool in the evaluation of cardiac blood flow rates. In Doppler ultrasound imaging, a reflection from a stationary object provides a signal at zero frequency (that is, at the intermediate frequency). The Doppler frequency shift in the echo signal returned from a moving target varies monotonically with the instantaneous velocity of the target. A review of cardiac Doppler measurement technology is contained in R. G. O'Connell, Jr., "The Role of Doppler Ultrasound in Cardiac Diagnosis," Hewlett-Packard Journal, June 1986 at 20-25; in P. A. Magnin, "Doppler Effect: History and Theory," id. at 26-31; in L. I. Halberg et al, "Extraction of Blood Flow Information Using Doppler-Shifted Ultrasound," id. at 35-40; and in B. F. Hunt et al, "Digital Processing Chain for A Doppler Ultrasound Subsystem," id. at 45-48.
A typical prior art medical ultrasound imaging system employs a phased array transducer, a scanner unit and a signal processing and display unit. The scanner unit provides analog signal conditioning, beamforming and signal translation from the ultrasound range to a more convenient intermediate frequency (I.F.) range. The processing and display unit then converts the analog I.F. signals to digital form and processes the digital samples in order to facilitate extraction and display of desired information contained in the transducer output. The display and processing unit may provide both black and white (monochrome) and color imaging. The monochrome mode typically is used to show anatomic detail, with blood flow shown in the color mode. In a typical system, a two-dimensional monochrome image may show a sector-shaped scan region of a patient, displayed at a rate of approximately 30 frames per second. A color mode image may be overlaid on a portion (up to 100%) of the scanned sector, displacing the monochrome image. At each picture element on the display, either the monochrome signal or the color signal is displayed; alternatively, the two signals may be combined in some fashion.
The color image is typically a color-coded blood flow map, where the color coding indicates localized velocity and turbulence of blood flow. In an exemplary commercial system, velocity is shown in shades of red and blue, red indicating flow toward the transducer and blue indicating flow away from the transducer, or vice versa. Sometimes another color may be mixed in over a portion of the scale to focus attention on flows within selected ranges. The intensity and/or shading of the color represents the speed of the flow toward or away from the transducer. Shades of green are sometimes added to indicate turbulence.
While the ultrasound image provides a qualitative representation of the region of interest, it is frequently desirable to obtain quantitative measurements of vessel parameters, such as blood velocity, vessel diameter and vessel wall directions. In order to determine blood velocity, the angle between the ultrasound beam direction and the direction of the blood vessel must be determined. A method for adjustment of Doppler angle in ultrasound images is disclosed in European Patent Application No. 0,755,920 published May 28, 1997. This published application describes a vessel analysis algorithm for calculating the direction of the vessel and coordinates of the vessel walls in the vicinity of the cursor when the cursor is positioned inside of a vessel in the ultrasound image.
Known techniques for quantitatively determining parameters such as blood velocity and vessel diameter from ultrasound images have been relatively difficult to use and have been time-consuming. In addition, such techniques have required relatively skilled operators and do not produce consistent results. Accordingly, it is desirable to provide methods and apparatus for automated measurement and analysis of patient anatomy from ultrasound images which overcome the drawbacks of prior art techniques. In particular, it is desirable to reduce the amount of training necessary to obtain such measurements from ultrasound images, to reduce or eliminate the variability of results related to differences between ultrasound operators, to diminish the influence of screen resolution on measurement results, to reduce overall ultrasound measurement time and to decrease operator stress and workload.